1. Field of the Invention
The present invention relates to remote sensing devices and, more particularly, to a device and method for remote sensing involving the use of a control module and a remote device module optically coupled thereto whereupon changes in physical parameters detected by the device module are transmitted as data signals back to the control module. In one embodiment, the invention concerns a device and method for remote sensing by multiplexing information from a plurality of binary optical sensors that detect the operating states of corresponding switches and controls.
2. Description of the Prior Art
Remote sensing of the operating status of switches has broad utility, particularly in the manufacturing arts. In such applications, switches indicative of the operation or parameters of the various production equipment need to be monitored to insure accuracy and speed of operation. Any malfunction of the manufacturing of equipment can be monitored by such remote sensing techniques so that an alarm signal can be generated to insure that remedial action is taken quickly. Such sensing is also useful in automated manufacturing operations, e.g., to control parameters of the line equipment, insure that operating instructions are carried out, adjust speed or other factors in response to detected situations, etc. In these applications, the remote sensing equipment is typically coupled to a computer or microprocessor controller so that responsive action can be taken automatically according to the detected line operation.
Devices for performing remote sensing are known and typically comprise a control box that is located remote from the monitored switches. Information concerning the status of these switches is conveyed to the control box via appropriate means. For example, switches incorporating optical sensors are known wherein operation of the switch selectively blocks and unblocks light that is carried via an optical fiber. In such sensing arrangements, the switch typically transmits at least 3-4 times more light in one position than in another, e.g., 10-12% light transmission when "ON" and 3-4% transmission when "OFF." Conventional systems may therefore employ a plurality of fiber optic cables coupling the switches to a remote control module so that the operational status of each switch can be monitored. Devices for multiplexing such numerous signals are well known, but require connection to local electrical power sources rather than using a power supply derived from an optical fiber.
An improvement in this arrangement is known in which the number of optical fibers coupled to the control module is reduced by employing other modules--called "device modules"--that are located relatively close to the monitored switches. The device modules are each connected via a splitter to a single optic fiber that is ultimately coupled to the remote control module. Such a device is illustrated in a publication authored by Fasching et al. and which is available from the NTIS under Publication No. DE85 017761.
In the Fasching et al. device, the central optical fiber is provided with photodiodes at each end thereof. Light generated by the control module is transmitted via the optical fiber to the device modules where it is converted via a photodiode and voltage multiplier to an 8 to 15 volt level. This voltage is used by circuitry in the device module to determine the state of an analog sensor via a resistive bridge, as well as to power the circuitry contained in the device module for obtaining this information. The obtained information is then converted via the same photodiode into a light signal that is transmitted back to the control module on the optical fiber.
A drawback of the Fasching et al. device is the need for providing a separate device module for each sensor that is to be monitored. This raises the cost and burden of installing such a system when a plurality of sensors are involved. A further drawback is the need for relatively high intensity light being generated and transmitted on the common optical fiber due to the use of relatively high (8-15 volts) levels in the device modules. This is further complicated by the large number of device modules needed to accommodate numerous sensors. A further drawback of the Fasching et al. device relates to the lack of information transmitted from the device modules to the control module regarding status of the interconnecting fiber optic cable, e.g., whether a break has occurred permitting introduction of ambient light.
Accordingly, a need exists for a remote sensing device capable of determining the operating status of numerous binary optical sensors without the drawbacks outlined above.
More broadly stated, a need generally exists for a remote sensing device utilizing optical coupling between the control end and the remote device end wherein power for any circuitry contained in the remote device end is provided via the optical coupling. Although Fasching et al. shows delivery of power in such a manner, the prior art device suffers in requiring immediate multiplication of the power in order to operate circuit elements within the remote modules. This arrangement suffers in that it requires high power consumption, and hence relatively high light transmission, to ensure operation of the circuits within the remote modules. A need thus exists for a remote optical sensing arrangement whereby lower power consumption, i.e., operation without the use of power multipliers, is present.
A further need exists with respect to monitoring of the optical sensing system, particularly with respect to operation of the remote module circuitry as well as integrity of the optical interconnection. Through such monitoring, accurate and reliable operation of the remote sensing system can be assured. In comparison, the lack of such monitoring raises doubts as to whether data being received by the control module is accurate as to the status of the physical parameter(s) being sensed or whether it is in fact indicative of a fault occuring within the system.
Another source of failure in prior art optical sensing systems is the introduction of ambient light into the binary optical sensors and/or their respective optical connections. Such ambient light will typically cause false data to be generated by the remote module, thus leading to errors in data interpretation carried out by the control module. One prior art approach to avoiding errors due to ambient light involves modulating the signal light at a known frequency, e.g., a few kilohertz, and then examining the modulated content of the return light. Such known arrangements cannot be employed in low power systems, however. A need therefore exists for a means to identify and reject such false ambient light signals in remote sensing devices.